Pressure sensors or gas sensors for use in a flammable/explosive atmosphere such as factories and chemical plants are required to employ a structure meeting construction requirements for electrical equipment for explosive atmospheres. In the construction requirements for explosive atmospheres, various requirements are set for each of an intrinsic safety explosion-proof structure, a pressure-resistant explosion-proof structure, an increased safety explosion-proof structure, or the like. The above sensors preferably employ the intrinsic safety explosion-proof structure that does not trigger explosion as an explosion-proof structure. In the case of the intrinsic safety explosion-proof structure, however, a value of current flowing through equipment is limited, and even general sensors that operate by dry batteries of a few volts, for example, exceed the current limit for the intrinsic safety explosion-proof structure. Thus, it is difficult to employ the intrinsic safety explosion-proof structure. Japanese Patent Laid-Open Publication No. 2009-54940 discloses a pressure-resistant explosion-proof structure for information terminals that allows transmission and reception of information by use of optical communication such as infrared ray by accommodating equipment in a case having an explosion-proof structure in order to externally transmit and receive information in an explosion-proof atmosphere.
Flammable substances include a liquid such as petroleum and gas such as LPG and LNG. Highly-volatile petroleum evaporates even at a normal temperature, and its vapor or gas is mixed with the air to be possibly ignited and exploded by electrostatic sparks or faint electric sparks generated when a switch or the like is turned ON/OFF. Here, explosion means combustion accompanied by rapid changes in propagation speed, pressure and temperature. Also, combustion means a chemical reaction in which a substance combines with oxygen in the air, emitting light and heat, and requires a flammable substance, oxygen, and thermal energy such as a flame, electric spark, friction heat and reaction heat necessary for ignition.
The explosion-proof structures further include a pressurized explosion-proof structure that prevents a flammable substance from entering a case from outside by enclosing clean air or a nonflammable gas such as nitrogen within the case and thereby setting a pressure therein to be higher than that of the outside. However, when general sensors are used, an air flow or the like may affect measurement. Thus, it is preferable to employ a structure in which flame does not leak outside a case that accommodates the sensors even when combustion or explosion occurs inside the case, and an ignitable gas around the case, if any, is not ignited (the pressure-resistant explosion-proof structure).
There has been known a canned motor pump in which a special pressure switch is provided in a terminal box, as disclosed in HERMETIC-Pumpen GmbH, PRODUCT INFORMATION, “Single-stage canned motor pumps complying with the chemical standards”, [online], Aug. 19, 2010, Internet, <URL: http://www.hermetic-pumpen.com/en/hermetic/products.html>. FIG. 7 is a sectional view of a terminal portion 100, and shows a terminal box 116 that is fitted with a terminal flange 117 connected to a sealed space of a stator portion. In the terminal flange 117, a stator coil connection line 111 extending from the stator portion is connected to a terminal 112, and a pressure switch 113 is arranged in the vicinity of the terminal 112. The pressure switch 113 has a mechanism whereby a diaphragm is deformed to connect an electric contact when a pressure reaches a preset value. Terminals 114 and 115 of the electric contact are arranged in the terminal box, and a pressure switch connection line is guided to outside from a pressure switch connection line mounting tool 119 of the terminal box 116. Similarly, the stator coil connection line 111 connected to the terminal is guided to outside from a coil connection line mounting tool 118.
In a canned motor pump, an inner portion of a stator of a motor that drives a centrifugal pump is covered by a can, and a handled liquid (for example, a flammable liquid) is filled therebetween, so that a rotating portion of the pump is immersed in the handled liquid and thus not required to be sealed. Also, since the stator is hermetically sealed by the can, a motor outer cylinder and an end bell, a sealed space is formed in the stator portion. Even when the can is damaged or the like, the liquid does not leak outside but enters the inner portion of the stator.
When the handled liquid enters the inner portion of the stator, insulation failure may occur in a stator coil to thereby damage the stator coil. To solve the problem, a pressure switch is provided in the sealed space of the stator portion, to detect a pressure increase due to the liquid leakage of the handled liquid and thereby determine that the liquid is leaking, as disclosed in HERMETIC-Pumpen GmbH, PRODUCT INFORMATION, “Single-stage canned motor pumps complying with the chemical standards”, [online], Aug. 19, 2010, Internet, <URL: http://www.hermetic-pumpen.com/en/hermetic/products.html>.
However, when only a small amount of liquid leaks to cause little pressure increase, the pressure switch, as disclosed in HERMETIC-Pumpen GmbH, PRODUCT INFORMATION, “Single-stage canned motor pumps complying with the chemical standards”, [online], Aug. 19, 2010, Internet, <URL: http://www.hermetic-pumpen.com/en/hermetic/products.html>, cannot accurately detect the liquid leakage. To detect such a small amount of liquid leakage, a highly-accurate electrical pressure sensor or a gas sensor needs to be used. When the pressure switch is replaced with the electrical pressure sensor or the gas sensor, the conventional terminal flange 117 including a mounting base needs to be replaced for each sensor, thereby causing an increase in cost.
Thus, it is an advantage of the present invention to provide a pressure-resistant explosion-proof connector that allows even a different type of sensor to be mounted via a common connector and thereby eliminates the need for replacement of a connector including a mounting base for each sensor.